The recovery of hydrocarbons from subterranean zones relies on the process of drilling wellbores. The process includes drilling equipment situated at surface and a drill string extending from the surface equipment to the formation or subterranean zone of interest. The drill string can extend thousands of meters below the surface. The terminal end of the drill string includes a drill bit for drilling (or extending) the wellbore. In addition to this conventional drilling equipment the system also relies on some sort of drilling fluid, which in most cases is a drilling “mud” which is pumped through the inside of the drill string. The drilling mud cools and lubricates the drill bit and then exits out of the drill bit and carries rock cuttings back to surface. The mud also helps control bottom hole pressure and prevents hydrocarbon influx from the formation into the wellbore, which can potentially cause a blow out at surface.
Directional drilling is the process of steering a well away from vertical to intersect a target endpoint or follow a prescribed path. At the terminal end of the drill string is a bottom-hole-assembly (“BHA”) which comprises 1) a drill bit; 2) a steerable downhole mud motor of rotary steerable system; 3) sensors of survey equipment (logging-while-drilling (LWD) and/or measurement-while-drilling (MWD)) to evaluate downhole conditions as well depth progresses; 4) equipment for telemetry of data to surface; and 5) other control mechanisms such as stabilizers or heavy weight drill collars. The BHA is conveyed into the wellbore by a metallic tubular.
MWD equipment is used to provide downhole sensor and status information to surface in a near real time mode while drilling. This information is used by the rig crew to make decisions about controlling and steering the well to optimize drilling speed and trajectory based on numerous factors including lease boundaries, location of existing wells, formation properties, and hydrocarbon size and location. This can include making intentional deviations from an originally-planned wellbore path as necessary based on information gathered from the downhole sensors during the drilling process. The ability to obtain real time data during MWD results in a relatively more cost effective and efficient drilling operation.
Known MWD tools contain essentially the same sensor package to survey the wellbore, however the data may be sent back to surface by various telemetry methods. Such telemetry methods include, but are not limited to the use of a hardwired drill pipe, acoustic telemetry, use of a fibre optic cable, mud pulse (MP) telemetry and electromagnetic (EM) telemetry. The sensors are usually located in an electronics probe or instrumentation assembly contained in a cylindrical cover or housing located near the drill bit.
MP telemetry involves creating pressure waves in the drilling mud circulating inside the drill string. Mud circulates from surface to downhole using positive displacement pumps. The resulting flow rate of mud is typically constant. Pressure pulses are generated by changing the flow area and/or flow path of the drilling mud as it passes the MWD tool in a timed, coded sequence, thereby creating pressure differentials in the drilling mud. The pressure differentials or pulses may be either negative pulses or positive pulses. Valves that open and close a bypass mud stream from inside the drill pipe to the wellbore annulus create a negative pressure pulse. All negative pulsing valves need a high differential pressure below the valve to create a sufficient pressure drop when the valve is open, but this results in negative valves being more prone to washing. With each actuation, the valve hits against the valve seat to ensure it completely closes the bypass; this impact can lead to mechanical and abrasive wear and failure. Valves that use a controlled restriction within the circulating mud stream create a positive pressure pulse. Some positive pulsing valves are hydraulically powered to reduce the required actuation power and typically have a main valve indirectly operated by a pilot valve. The pilot valve closes a flow restriction which actuates the main valve to create a pressure drop. Pulse frequency is typically governed by pulse generating motor speed changes. The pulse generating motor requires electrical connectivity with other elements of the MWD probe such as a battery stack and sensors.
In mud pulser systems, as well as in other downhole tools, the pulse generating motor driveline system is subjected to extreme pressure differentials of about 20,000 psi between external and internal aspects of the tool. To accommodate this large pressure differential, the drilling mud has access to areas of the tool which are positioned on one side of a pressure compensation mechanism. Pressure is equalized on the other side of the pressure compensation mechanism within the tool using clean, non-drilling fluid such as hydraulic fluid or silicon oil. Various mechanisms have been used to provide pressure compensation including metallic bellows, rubber compensation membranes, and piston compensation with springs. Given the large temperature differentials from surface to downhole, especially in colder drilling climates, there is a high chance of temperature related failures for MWD tool components, in particular rubber membranes used for pressure compensation.
A pressure compensation device described in WO 2012/130936 utilizes pistons and fluid to provide pressure compensation via a dual section chamber within a housing. The device allows fluid communication through borehole ports to prevent collapse or bulging of the pressure compensation device resulting from thermal expansion of the hydraulic fluid contained in one of the sections of the chamber. The pressure compensation device described in WO 2010/138961 includes a metal membrane that can accommodate large oil volumes. The metal membrane is capable of elastic deformation and is shaped to optimize such deformation in a desired manner to compensate for temperature and pressure effects experienced downhole. U.S. Pat. No. 8,203,908 describes a mud pulser system in which the spline shaft is surrounded by lubricating fluid which is pressurized against downhole hydrostatic pressure using a bellows style pressure compensator. In addition to a bellows seal, the system has a dual seal which maintains the integrity of the lubrication chamber during operation and replacement of the bellows seal during maintenance.